Method for removing heavy metals from an aqueous solution with cross-linked copolymers

ABSTRACT

Cross-linked cyclocopolymers made up of one or more quaternary ammonium salts and sulfur dioxide as monomers. One of the quaternary ammonium salts is also an aspartic acid derivative. The cross-linked copolymers include a repeating unit with multiple chelating centers that different metal ions can bind to. The cross-linked copolymers are zwitterionic or anionic, and can be in either an acidic form or a basic form. A method for removing metal ions from an aqueous sample with these cross-linked copolymers is also described.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of Ser. No. 14/918,722.

STATEMENT OF FUNDING ACKNOWLEDGEMENT

This project was funded by the National Plan for Science, Technology andInnovation—King Abdulaziz City for Science and Technology—the Kingdom ofSaudi Arabia, award number (11-ADV2132-04).

BACKGROUND OF THE INVENTION Technical Field

The present invention relates to polymers. More particularly, thepresent invention relates to cross-linked copolymers containingpolymerized units of one or more quaternary ammonium salt monomers.These polymers are suitable for water treatment applications,specifically removal of heavy metal ions by adsorption.

Description of the Related Art

The “background” description provided herein is for the purpose ofgenerally presenting the context of the disclosure. Work of thepresently named inventors, to the extent it is described in thisbackground section, as well as aspects of the description which may nototherwise qualify as prior art at the time of filing, are neitherexpressly or impliedly admitted as prior art against the presentinvention.

Heavy metals are released into the surface and ground water because ofvarious activities such as electroplating, and pigment and paintmanufacturing. Because of their toxicity and tendency to bioaccumulate,the removal of metals from industrial effluents before discharge intothe environment is required to mitigate any impact on plants, animalsand humans [Heidari, A., H. Younesi, and Z. Mehraban, Removal of Ni(II),Cd(II), and Pb(II) from a ternary aqueous solution by aminofunctionalized mesoporous and nano mesoporous silica. ChemicalEngineering Journal, 2009. 153(1-3); p. 70-79 (reference), incorporatedherein by reference in its entirety]. Lead is one of the most toxicmetals that are widely used in various industries, such as battery andglass manufacturing, metal plating and finishing, printing and tanning.The permissible levels of lead in drinking and waste water are 0.05 mg/Land 0.005 mg/L, respectively [(EPA), E.P.A., Environmental PollutionControl Alternatives. 1990 (EPA/625/5-90/0255 CPA/625/4-89/023)(reference), incorporated herein by reference in its entirety]. Severalconventional methods are used for the removal of pollutants [Jiang,M.-q., et al, Removal of Pb(II) from aqueous solution using modified andunmodified kaolinite clay. Journal of Hazardous Materials, 2009. 170(1):p. 332-339 (reference), incorporated herein by reference in itsentirety]. However, these technologies are either expensive for thetreatment and disposal of the secondary toxic sludge or ineffective whenthe toxic metal is present in wastewaters at low concentrations [Rao, M.M., et al., Removal of some metal ions by activated carbon prepared fromPhaseolus aureus hulls. Journal of Hazardous Materials, 2009. 166(2-3);p. 1006-1013 (reference), incorporated herein by reference in itsentirety].

Alternatively, heavy metals in wastewater are removed by adsorptionwhich is both efficient and relatively simple. A successful adsorptionprocess depends on the adsorption performance of the adsorbents. Variousconventional adsorbents have been reported for the removal of lead fromwastewaters including activated carbon, clay, metal oxides nanoparticlesand nanomaterials [Ghaedi, M., et al., Comparison of the efficiency ofpalladium and silver nanoparticles loaded on activated carbon and zincoxide nanorods loaded on activated carbon as new adsorbents for removalof Congo red from aqueous solution: Kinetic and isotherm study.Materials Science and Engineering: C, 2012. 32(4): p. 725-734; Dias, J.M., et al., Waste materials for activated carbon preparation and its usein aqueous-phase treatment: A review. Journal of EnvironmentalManagement, 2007. 85(4): p. 833-846; Erdem, E., N. Karapinar, and R.Donat, The removal of heavy metal cations by natural zeolites. Journalof Colloid and Interface Science, 2004. 280(2): p. 309-314 (references),each incorporated herein by reference in their entirely”. However, smallparticle size of nanoparticle results in the difficulty of separationfrom solution, which limits the application in water treatment. The newadsorbents requested by the industry should have high capacity, rapidadsorption kinetics and operational stability at elevated temperaturesin the presence of steam and other reaction components. The newadsorption processes may then take advantage of such materials.

Polymers could represent good adsorbent candidates displaying apronounced chemical versatility given by the great number of chemicalfunctionalities or motifs present in their structures. Recently,researchers have focused on the syntheses of zwitterionic cross-linkedinorganic and/or organic hybrid polymer materials for the removal ofheavy metal ions via electrostatic effects [Liu, J., et al., Novelnegatively charged hybrids. 3. Removal of Pb²⁺ from aqueous solutionusing zwitterionic hybrid polymers as adsorbent. Journal of HazardousMaterials, 2010. 173(1-3): p. 438-444; Liu, J., et al., Preparation ofzwitterionic hybrid polymer and its application for the removal of heavymetal ions from water. Journal of Hazardous Materials, 2010. 178(1-3):p. 1021-1029; Liu, J., et al., Novel negatively charged hybrids. 1.copolymers: Preparation and adsorption properties. Separation andPurification Technology, 2009. 66(1): p. 135-142 (references), eachincorporated herein by reference, in their entirety]. Considerableattention has been given to synthesize chelating agents containing anamino methyl phosphonate motif owing to its extraordinary chelatingproperties in extracting heavy metal ions from wastewater. Morerecently, a porous resin with Schiff base chelating groups for removalof heavy metal ions from aqueous solutions has been synthesized[Ceglowski, M. and G. Schroeder, Preparation of porous resin with Schiffbase chelating groups for removal of heavy metal ions from aqueoussolutions. Chemical Engineering Journal, 2015. 263(0): p. 402-411(reference), incorporated herein by reference in its entirety].

In view of the foregoing, there exists a need for novel materials andcompositions with high adsorption capacity for Pb²⁺ and advantageously,a range of other metals, over a short equilibrium time.

BRIEF SUMMARY OF THE INVENTION

According to a first aspect, the present disclosure relates to acopolymer comprising an aspartic acid derivative, a quaternary ammoniumsalt and sulfur dioxide. The aspartic acid derivative, the quaternaryammonium salt and the sulfur dioxide are cyclopolymerized to form thecopolymer. In the copolymer, a sulfur dioxide molecule connects everytwo molecules of the aspartic acid derivative, the quaternary ammoniumsalt, or both.

In one or more embodiments, the quaternary ammonium salt cross-links onepolymer chain of the copolymer to another polymer chain of thecopolymer.

In some embodiments, the aspartic acid derivative comprises a quaternarynitrogen atom.

In certain embodiments, the aspartic acid derivative is represented byFormula A:

where R₁ is a hydrogen, a halide or an optionally substituted C₁-C₆alkyl group and X⁻ is F⁻, Cl⁻, Br⁻, I⁻, NO₃ ⁻ or other suitablemonoanion other than OH⁻.

In one embodiment, R₁ is a hydrogen and X⁻ is Cl⁻.

In some embodiments, the quaternary ammonium salt is represented byFormula B:

where R₂-R₅ are each independently a hydrogen, a halide or an optionallysubstituted C₁-C₆ alkyl group and Y⁻ and Z⁻ are each F⁻, Cl⁻, Br, I⁻,NO₃ ⁻ or other suitable monoanion other than OH⁻.

In one embodiment, R₂-R₅ are each a hydrogen and Y⁻ and Z⁻ are each Cl⁻.

In certain embodiments, the copolymer is in a basic and anionic form.

In some embodiments, the copolymer has a repeating unit represented byFormula 1 or Formula 2:

where R₁-R₅ are each independently a hydrogen, a halide, an optionallysubstituted methyl group, or an optionally substituted ethyl group; eachW⁻ in Formula 1 is F⁻, Cl⁻, Br, I⁻, NO₃ ⁻ or other suitable monoanionother than OH⁻; each M⁺ in Formula 2 is Li⁺, Na⁺, K⁺ or other suitablemonocation other than H⁺; and n=9-19.

In one embodiment, the repeating unit of the copolymer is represented byFormula 1, where R₁-R₅ are each a hydrogen and W⁻ is Cl⁻.

In another embodiment, tire repeating unit of the copolymer representedby Formula 2, where R₁-R₅ are each a hydrogen and M⁺ is Na⁺.

In some embodiments, the copolymer has a Pb²⁺ adsorption capacity of50-100 mg g⁻¹ based on a total weight of the copolymer.

According to a second aspect, the present disclosure relates to a methodfor making the copolymer. The method comprises cyclopolymerizing theaspartic acid derivative, the quaternary ammonium salt and the sulfurdioxide in the presence of an initiator and a non-aqueous solvent toform a cross-linked polyzwitterionic acid polymer and optionallytreating the polyzwitterionic acid polymer with a base to form across-linked anionic polyelectrolyte polymer.

In one embodiment, the initiator is azoisobutyronitrile and thenon-aqueous solvent is dimethylsulfoxide.

According to a third aspect, the present disclosure relates to a methodfor removing Pb²⁻ from an aqueous sample. The method comprisescontacting the aqueous sample with the copolymer to adsorb Pb²⁻ from theaqueous sample onto the copolymer.

In one or more embodiments, the copolymer has a concentration of0.02-5.0 g L⁻¹ in the aqueous sample.

In some embodiments, the contacting is carried out at a temperature of10-100° C.

In certain embodiments, the contacting is carried out at an aqueoussample pH range of 3 to 9.

In certain embodiments, the contacting is carried out for 5-180 min.

In one embodiment, the contacting removes more than 90% of the Pb²⁺present in the aqueous sample.

The foregoing paragraphs have been provided by way of generalintroduction, and are not intended to limit the scope of the followingclaims. The described embodiments, together with further advantages,will be best understood by reference to the following detaileddescription taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the disclosure and many of the attendantadvantages thereof will be readily obtained as the same becomes betterunderstood by reference to the following detailed description whenconsidered in connection with the accompanying drawings, wherein:

FIG. 1 is a scheme illustrating synthesis of a crosslinkedpolyzwitterionic acid or a crosslinked dianionic polyelectrolyteaccording to one embodiment.

FIG. 2 is a curve illustrating the effect of pH on the adsorption ofPb²⁺ ions.

FIG. 3 is a series of curves illustrating the effect of Pb²⁺concentration on Pb²⁺ removal.

FIG. 4 is a line graph illustrating the effect of Pb²⁺ initialconcentrations on the adsorption capacity.

FIG. 5A is an SEM image of an unloaded crosslinked dianionicpolyelectrolyte (CAPE).

FIG. 5B is an EDX spectrum of the unloaded crosslinked dianionicpolyelectrolyte (CAPE) of FIG. 5A.

FIG. 6A is an SEM image of a Pb²⁺-loaded crosslinked dianionicpolyelectrolyte (CAPE).

FIG. 6B is an EDX spectrum of the Pb²⁺-loaded crosslinked dianionicpolyelectrolyte (CAPE) of FIG. 6A.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Referring now to the drawings, wherein like reference numerals designateidentical or corresponding pails throughout the several views.

The present disclosure provides cross-linked copolymers where one ormore of the monomers constituting the copolymer each contain one or morequaternary nitrogen atoms. These quaternary ammonium salt monomers areneutral and zwitterionic, where positive and negative electrical chargesof 1 to 2 are present in each monomeric molecule at equal amounts. Insome embodiments, the copolymer has more than one quaternary ammoniumsalt monomer where at least one of the monomers is an aspartic acidderivative monomer. In some embodiments, at least one of the quaternaryammonium salt monomers is also a cross-linking agent for the copolymer.The cross-linked copolymer macromolecule, formed by cyclopolymerizationof the different monomers, can exist in an acidic form or a basic form.Preferably, the copolymer is in a basic form.

Monomers constituting a copolymer provided herein are represented byFormula A, Formula B and Formula C:

where R₁-R₅ are each independently a hydrogen, a halide, an optionallysubstituted C₁-C₆ alkyl group, preferably a methyl group, or anoptionally substituted ethyl group, preferably a hydrogen; X⁻ is F⁻,Cl⁻, Br⁻, I⁻, NO₃ ⁻ or other suitable monoanion other than OH⁻,preferably a halide, more preferably Cl⁻; and when cyclopolymerized intoa copolymer, every two monomers represented by Formula A, Formula B, orboth are connected by a sulfur dioxide molecule. In other words, incertain embodiments, each monomer represented by Formula A or Formula Bis sulfonized prior to or during the cyclopolymerization.

In one embodiment, the monomers of the copolymer includeN,N-diallylaspartic acid hydrochloride,1,1,4,4-tetrallylpiperazine-1,4-diium dichloride and sulfur dioxide.

In certain embodiments, a copolymer of the present disclosure, inaddition to the monomers represented by Formulas A, B and sulfurdioxide, further include at least one selected from the group consistingof a monomer having a mono-, di-, tri- or tetraallyl group; a monomerhaving one or more carboxylic acid functional groups; a monomercontaining one or more quaternary nitrogen atoms that are optionallypart of a 3- to 8-membered heterocyclic ring; nitrogen dioxide; nitrogendisulfide; carbon dioxide; and carbon disulfide.

For purposes of the present invention, “cross-linked” or “network” or“thermoset” polymers refer to natural or synthetic polymers and resinsthat contain branches that connect polymer chains via covalent bonds.The cross-linking can alter the physical and mechanical properties ofthe polymer. The vulcanization of rubber, for example, results from theintroduction of short chains of sulfur atoms that link the polymerchains in natural rubber. As the number of cross-links increases, thepolymer becomes more rigid.

Cross-links can be formed by chemical reactions that are initiated byheat, pressure, change in pH, or radiation, with or without the presenceof a cross-linking agent and/or a catalyst.

In one or more embodiments, one of the monomers acts as a cross-linkingagent and is therefore a cross-linking monomer. In one embodiment, thecross-linking monomer is the quaternary ammonium salt monomerrepresented by Formula B, and each polymer chain of a copolymerdescribed herein contains 5-10% of this monomer based on a total numberof monomers represented by Formulas A and B, preferably 5-8%, morepreferably 6-8%. In one embodiment, each polymer chain of the copolymercontains 7% of the monomer represented by Formula B relative to a totalnumber of monomers represented by Formulas A and B. In one embodiment,the cross-linking quaternary ammonium salt monomer is1,1,4,4-tetrallylpiperazine-1,4-diium dichloride.

For purposes of the present invention, “quaternary ammonium salt”, whichis also called “quaternary ammonium compound” or “quaternary amine”,refers to a salt having one or more quaternary ammonium cations with ananion. Quaternary ammonium cations are positively charged polyatomicions with a generic formula of NR₄ ⁺, with R being the same or differentalkyl or aryl groups.

For purposes of the present invention, “cyclopolymerization” or“cyclocopolymerization” refers to a polymerization reaction where one orsnore ring structures, heterocyclic or homocyclic, are formed.

For purposes of the present invention, a “repeat unit” or “repeatingunit” is a part of a polymer or a resin whose repetition would producethe complete polymer chain (except for the end-groups) by linking therepeat units together successively along the chain.

The cross-linked copolymer of the present disclosure has a repeatingunit that is represented by Formula 1 or Formula 2:

where:

R₁-R₅ are each independently a hydrogen, a halide, an optionallysubstituted C₁-C₆ allyl group, preferably a methyl group, or anoptionally substituted ethyl group, preferably a hydrogen; W⁻ in Formula1 is F⁻, Cl⁻, Br⁻, I⁻, NO₃ ⁻ or other suitable monoanion other than OH⁻,preferably a halide, more preferably Cl⁻; and M⁺ in Formula 2 is Li⁺,Na⁺, K⁺ or other suitable monocation other than H⁺, preferably an alkalimetal, more preferably Na⁺; n=9-19.

The repeating unit can be repeated in the cross-linked copolymermacromolecule 10 to 10000 times, preferably 50 to 5000 times, morepreferably 20 to 2500 times, 25 to 1500 times, 100 to 1000 times.

In certain embodiments, a copolymer of the present disclosure issynthesized using co-cyclopolymerization protocols as described in theliterature with slight modifications as recognized as appropriate by aperson of ordinary skill in the polymer chemistry art [Butler, G. B.,Cyclopolymerization and cyclocopolymerization. Marcel Dekker, New York1992: P. K. Singh, V. K. S., M. Singh, E-Polymers, 2007. 030: p. 1-34;S. Kudaibergenov, W. J., A. Laschewsky, Adv. Polym. Sci., 2006. 201: p.157-224; and Jaeger, W., J. Bohrisch, and A. Laschewsky, Syntheticpolymers with quaternary nitrogen atoms—Synthesis and structure of themost used type of cationic polyelectrolytes. Progress in PolymerScience, 2010. 35(5): p. 511-577; Ali, S. A. and O. C. S. Al-Hamouz,Comparative solution properties of cyclooopolymers having cationic,anionic, zwitterionic and zwitterionic/anionic backbones of similardegree of polymerization. Polymer, 2012, 53(15): p. 3368-3377;Abu-Thabit, N.Y., et al., Phosphonobetaine/sulfur dioxide copolymer byButler's cyclopolymerization process. European Polymer Journal, 2011.47(5): p. 1113-1123; Ali, S. A., et al, Synthesis and comparativesolution properties of single-, twin-, and triple-tailed associatingionic polymers based on diallylammonium salts. Journal of PolymerScience Part A: Polymer Chemistry, 2006, 44(19): p. 5480-5494; Butler,G. B., Cyclopolymerization. Journal of Polymer Science Part A: PolymerChemistry, 2000. 38(19): p. 3451-3461; McGrew, F. C., J. Chem., 1958.35: p. 178-186 (references), each incorporated herein by reference intheir entirety”.

In a non-limiting example, a cross-linked copolymer can be synthesizedby initially dissolving an aspartic acid derivative monomer and across-linking quaternary ammonium salt monomer in a non-aqueous solventto form a polymer solution. Preferably, the non-aqueous solvent is apolar, aprotic solvent selected from but not limited to tetrahydrofuran(THF), ethyl acetate, acetone, dimethylformamide (DMF), acetonitrile(MeCN), dimethyl sulfoxide (DMSO), nitromethane and propylene carbonate.In one embodiment, the non-aqueous solvent is DMSO. The polymersolution, as prepared, contains the aspartic acid derivative monomer andthe cross-linking quaternary ammonium salt monomer at a molar ratio of10-20:1, preferably 10-15:1, more preferably 12-14:1. Sulfur dioxide isadded to the polymer solution, for example, by gas absorption such thatthe polymer solution contains sulfur dioxide, the aspartic acidderivative monomer and the cross-linking quaternary ammonium saltmonomer at a molar ratio of 15-18:12-14:1, preferably 15-17; 13-14:1,more preferably 15-16:13-14:1. An initiator, azoisobutyronitrile is thenmixed with the polymer solution to a final concentration of 25-100 mM,preferably 50-80 mM, more preferably 60-80 mM, even more preferably70-80 mM, most preferably 75-80 mM. In alternative embodiments, adifferent initiator such as ammonium persulfate (APS),tetramethylethylenediamine (TEMED), riboflavin and TEMED may be used.The reaction mixture containing the different monomers and the initiatoris mechanically stirred at 45-70° C., preferably 55-65° C. under aninert gas for 18-30 h. In one embodiment, the reaction mixture isstirred at 60° C. under N₂ for 24 h. Within 3-5 h of the stirring, atransparent swelled gel is formed. At the end of the stirring, across-linked polyzwitterionic acid polymer is formed. The acidiccopolymer can be further basified by immersing and agitating the resin,for 3-8 h, in an alkaline solution (e.g. NaOH, KOH, ammonia) oradvantageously, a series of alkaline solutions to ensure a completeionic exchange, to form a cross-linked anionic polyelectrolyte polymerthat is in basic form. The copolymer in basic form is washed and rinsedwith water or a non-aqueous solvent, filtered and dried until the resinreadies a constant weight.

Each repeating unit in a cross-linked copolymer in the presentdisclosure, as shown in Formulas 1 and 2, includes multiple ligandcenters or chelating centers (i.e. central nitrogen atom of thequaternary ammonium group, COO²⁻ and also possibly X⁻, OH⁻¹) to whichone or more metal ions can be coordinated. The metal ions that arecoordinated to the plurality of ligand centers are preferably heavymetal ions, including but not limited to Ag⁺, Na⁺, Pb²⁺, Mn²⁺, Fe²⁺,Co²⁺, Ni²⁺, Cu²⁺, Sn²⁺, Cd²⁺, Hg²⁺, Cr³⁺, Fe³⁺, As³⁺, Sb⁵⁺ and Cr⁶⁺. Inone embodiment, the number of ligand centers in a repeating unit of thecopolymer, as represented by Formula 1 or Formula 2 and depending on thevalue of n, is 5 to 100, preferably 15 to 75, more preferably 25 to 50.Each metal atom is coordinated by a ligand center in a monodentatemanner.

In view of the foregoing, a cross-linked copolymer according to thepresent invention possesses adsorption capacity towards a wide range ofmetal ions. The present disclosure further provides a method forremoving metal ions from an aqueous solution by contacting and adsorbingthe metal ions with the copolymer, in both batch mode and fixed-bed orcolumn mode. Examples of metal ions that can be adsorbed by thecross-linked copolymer are outlined above.

In one embodiment, a cross-linked copolymer according to the presentinvention is effective in removing Pb²⁺ ions from an aqueous sample. Theinitial concentration of Pb²⁺ ions in the aqueous solution (batch mode)is 5-500 ppm, preferably 10-100 ppm.

In one or more embodiments, the cross-linked copolymer is present in theaqueous sample within a concentration range of 0.02-5.0 g L⁻¹ (pervolume of the treated aqueous solution), preferably 0.5-3.0 g L⁻¹, morepreferably 1.0-2.0 g L⁻¹. In one embodiment, an aqueous sample istreated with 1.5 g L⁻¹ of the copolymer.

In one or more embodiments, the method for removing metal ions iscarried out at an aqueous sample pH range of 3-9, preferably 3-7, morepreferably 3-6. In one embodiment, the, the aqueous sample has a pH of6.0.

In one or more embodiments, a cross-linked copolymer of the present:disclosure is effective in adsorbing metal ions in an aqueous samplewithin a temperature of 10-100° C., preferably 20-80° C., morepreferably 25-60° C., most preferably 25-40° C.

In one or more embodiments, the adsorption of metal ions by across-linked copolymer of the present invention in an aqueous solutionis carried out for a duration of 5-180 min, preferably 10-120 min, morepreferably 20-90 min. More than 90% of the metal ions present in theaqueous solution will be successfully removed at the end of theadsorption process, preferably more than 95%, more preferably more than99%. Advantageously, more than 90% of the metal ions are removed withinthe first 15 min.

In at least one embodiment, the metal removal or adsorption process by across-linked copolymer of the present disclosure is an exothermicprocess, as indicated by a calculated negative ΔH value.

In at least one embodiment, the metal removal or adsorption process by across-linked copolymer of the present disclosure is a spontaneousprocess, as indicated by a calculated negative ΔG value.

In one or more embodiments, the cross-linked copolymer has a Pb²⁺adsorption capacity of 50-100 mg g⁻¹ based on the total weight of thecopolymer, preferably 50-75 mg g⁻¹, more preferably 60-70 mg g⁻¹.

In certain embodiments, a cross-linked copolymer according to thepresent disclosure can be regenerated and reused as a heavy metaladsorbent for at least 5 cycles with minimal decrease in adsorptionefficiency (no more than a 2% decrease in mercury removal with eachregeneration cycle). To regenerate the adsorbent, metal ions aredesorbed from a cross-linked copolymer and this can be achieved bytreating a spent copolymer resin, i.e. a metal-loaded copolymer resin inan acidic or basic solution of at least 0.5 M in concentration. Strongacids and bases such as HCl, H₂SO₄, HNO₃, NaOH and KOH are preferred,but organic acids, weak acids and weak bases (e.g. acetic acid, ammoniaetc.) may also be used for metal ion desorption process.

EXAMPLES

The examples below further illustrate protocols for preparing andcharacterizing the cross-linked copolymers described herein, and are notintended to limit the scope of claims.

In the following examples, a functionalized resin containing thederivatives of aspartic acid as monomers and where the monomers haveunquenched nitrogen valency was synthesized by a cyclopolymerizationprotocol. FTIR, EDX, TGA and SEM characterization techniques were usedto confirm the structural and morphological properties of thecross-linked dianionic polyelectrolyte (CAPE) resin. The resin displaysan outstanding capability to remove Pb²⁺ ions over a wide range of pHvalues. The contact time was found to have an effect upon Pb²⁺adsorption. The linearity of the plots t/qt vs. t implies the adsorptionfollowed the pseudo-second order rate kinetics with high adsorptioncapacity. Fitting the data to Langmuir, Freundlich and Temkin modelsshow that the Langmuir and Temkin models give a better correlationcoefficient with R² of 0.99. Thermodynamic parameters were evaluated;the negative values of ΔG° indicate the spontaneity and the negativevalues of ΔH° (−43.87 KJ/mol) showed the exothermic nature of Pb²⁺sorption on the CAPE resin. Based on the characterization results, theCAPE resin would find applications as a highly efficient adsorbent inremoval of heavy metal ions, especially lead ions from industrialwastewaters.

Example 1 Chemicals and Materials

Azoisobutyronitrile (AIBN) from Fluka AG was purified by crystallizationfrom a chloroform-ethanol mixture. Dimethylsulfoxide (DMSO) was driedover calcium hydride overnight and then distilled under reduced pressureat a boiling point of 64-65° C. (4 mm Hg). Standard solution (1000 mg/L)of Pb(II), hydrochloric acid, nitric acid and sodium hydroxide wereobtained from Sigma-Aldrich. The lead standard solution was utilized toprepare the required initial concentrations by dilution. All solventsused were of analytical grade.

Example 2 Physical Characterization Techniques

Field emission scanning electron microscope (FESEM) was used tocharacterize the surface morphology of the polymer before and after theadsorption of lead. Energy-dispersive X-ray spectroscope (EDX) equippedwith a detector model X-Max was employed to obtain the elementalspectrum and to get elemental analysis of the pristine polymer andPb(II)-loaded polymer. Thermo Scientific Ice 3000 flame atomicabsorption spectrometer (FAAS) equipped with a 10 cm air-acetyleneburner was used to monitor the concentration of Pb(II). Theconcentrations of some metal ions in real wastewater samples wasanalyzed by inductively coupled plasma mass spectrometry (ICP-MS) modelICP-MS XSERIES-II Thermo Scientific. IR spectra were recorded on aPerkin-Elmer 16F PC FTIR spectrometer. 1H and 13C spectra were measuredon a JEOL LA 500 MHz spectrometer using HOD signal at d4.65 and dioxanesignal at 67.4 ppm as internal standards, respectively.Thermogravimetric analysis (TGA) was carried out using a thermalanalyzer SDT Q600, V20.9 Build 20 manufactured by TA instruments, USA.The temperature was raised at a uniform rate of 10° C./min. The analyseswere made over a temperature range of 20-700° C. in an air-atmosphereflowing at a rate of 100 Ml/min.

The thermogravimetric analysis (TGA) curve of synthesized cross-linkeddianionic polyelectrolyte (CAPE) shows three distinct weight loss steps.The first slow but gradual weight loss of about 20% is attributed to theremoval of moisture and writer molecules embedded inside thecross-linked polymer. The second dramatic loss of about 25% around 320°C. is attributed to the loss of S₂O due to polymer degradation. Thethird slow and gradual loss of 15% is attributed to the combustion ofnitrogenous organics with the release of NOx, CO₂ and H₂O gases. At 700°C., the residual mass was found to be 40%.

Example 3 Synthesis of Cross-Linked Polyzwitterionic Acid (CPZA) andCross-Linked Dianionic Polyelectrolyte (CAPE)

To the best of the applicants, the following synthesis of the resinswould represent the first example of cross-linked cyclopolymercontaining the derivatives of aspartic acid as monomers by thecyclopolymerization protocol involving monomers having unquenchednitrogen valency. The cross-linker 1,1,4,4-tetrallylpiperazine-1,4-diiumdichloride was prepared as reported in literature [Ali, S. A., S. Z.Ahmed, and E. Z. Hamad, Cyclopolymerization studies of diallyl- andtetrallylpiperazinium salts. Journal of Applied Polymer Science, 1996.61 (7): p. 1077-1085 (reference), incorporated herein by reference inits entirety]. The monomer N,N-diallylaspartic acid hydrochloride wasprepared via Michael addition of diallylamine to dimethyl maleatefollowed by hydrolysis of the Michael adduct in aqueous NaOH andneutralization with aqueous HCl.

Referring to FIG. 1, the synthesis process began where, to a solution ofmonomer monomer N,N-diallylaspartic acid hydrochloride (6.06 g, 24.2mmol), cross-linker 1,1,4,4-tetrallylpiperazine-1,4-diium dichloride(0.582 g, 1.82 mmol) in DMSO (9.1 g) in a round bottom flask (50 ml),was absorbed SO₂ (1.78 g, 27.8 mmol) (from a cylinder) by gentle blowingit over the stirred surface of the solution. After the initiator AIBN(105 mg) was added, the reaction mixture was stirred at 60° C. under Nfor 24 h. Within 3-5 h, the magnetic stir-bar stopped moving; thereaction mixture became a transparent swelled gel. At the end of theelapsed time, the swelled gel of the cross-linked polyzwitterionic acid(CPZA) was soaked in water (48 h) with replacement of water severaltimes. The swelled gel in water (≈65 cm³) was agitated with NaOH (1.6 g,40 mmol) at room temperature for 5 h followed by further addition ofNaOH (1.6 g, 40 mmol) and stirring for 1 h to ensure complete exchangewith Na⁺. The CPZA in acid form is less expanded owing to thezwitterionic form while the anionic form in resin, cross-linkeddianionic polyelectrolyte, (CAPE), is highly expanded in the abovealkaline mixture. The CAPE resin was dropped onto acetone (200 ml),filtered, dried at 60° C. under vacuum to a constant weight (7.7 g,90%). (Found: C, 38.1; H, 4.6; N, 4.4; S, 9.5%). A terpolymer fromN,N-diallylaspartic acid hydrochloride in disodium form C₁₀H₁₃NNa₂O₄ (93mol %) and 1,1,4,4-tetrallylpiperazine-1,4-diium dichloride in thehydroxide form C₁₆H₃₀N₂O₂ (7 mol %) and SO₂ (100 mol %) requires C,38.65; H, 4.41; N, 4.62; S, 9.93).

Hence, N,N-diallylaspartic acid hydrochloride,1,1,4,4-tetrallylpiperazine-1,4-diium dichloride and SO₂ underwentcycloterpolymerization to give cross-linked polyzwitterionic acid (CPZA)which, upon basification with NaOH, afforded the cross-linked dianionicpolyelectrolyte (CAPE) in an excellent overall yield of 90%. The resinCAPE has the unquenched nitrogen valency, which can act as a chelationcenter along with the two carboxylate motifs. It is postulated that thethree basic centers (N and CO₂ ⁻) in aspartic acid with differentbasicity constant impart chelation properties that enable sequestrationof toxic metal ions.

Example 4 Adsorption of Pb²⁺ on Cross-Finked Dianionic Polyelectrolyte(CAPE)

A 30 mg of adsorbent CAPE was added in 20 mL of aqueous Pb²⁺ solution ofspecific concentration and then stirred for period of 2, 5, 10, 15, 20,30, 40, 50, 60, 90 and 120 minutes respectively at 298 K. This study wascarried out with different initial Pb²⁺ concentrations ranging from 10to 100 mgL⁻¹ while maintaining the adsorbent amount of 1.5 g L⁻¹. Theresultant solution was filtered using a filter paper and the filtratewas analyzed by atomic absorption spectroscopy to determine the amountof Pb²⁺ uptake. The pH of the solution was also measured during courseof adsorption. The effect of pH was studied at 298 K with an initialPb²⁺ concentration of 40 mgL⁻¹. The kinetic and thermodynamic behaviorswere studied in a similar manner with initial Pb²⁺ concentration of 40mgL⁻¹ at 298, 313 and 333 K respectively. The amount of Pb²⁺ adsorbed bythe adsorbent CAPE and the percentage removal of Pb²⁺ were computedusing the following Equations (1) and (2) respectively.

$\begin{matrix}{q_{t} = \frac{\left( {C_{i} - C_{t}} \right)V}{W}} & (1) \\{{\%{Pb}^{2 +}\mspace{14mu}{uptake}} = {\left( \frac{C_{i} - C_{t}}{C_{i}} \right) \times 100}} & (2)\end{matrix}$Here, C_(i) and C_(t) are the initial and final concentrations of Pb²⁺ions in mg L⁻¹ respectively; V is the volume of solution in L with whichthe resin of weight W in gram is contacted. The adsorption capacity atvarious times and equilibrium are denoted as q_(t) and q_(e),respectively, where in the case of q_(e), the equilibrium concentrationC_(e) is used instead of C_(t).

Example 5 Fourier Transform Infrared Spectroscopy of Cross-LinkedDianionic Polyelectrolyte (CAPE)

The FT-IR spectra of the resin CAPE before and after adsorptionexperiments were examined. The unloaded resin (30 mg) was contacted with40 mgL⁻¹ Pb²⁺ concentration at adsorbent amount of 1.5 g L⁻¹ for 120minutes at a pH of 6.0. It was filtered and dried under vacuum untilconstant weight was achieved.

The IR spectrum of cross-linked polyzwitterionic acid (CPZA) showsstrong bands at 1727 cm⁻¹ and 1631 cm⁻¹ which are usually attributed tothe asymmetric and symmetric stretchings of COOH [Al-Muallem, H. A., M.I. M. Wazeer, and S. A. Ali, Synthesis and solution properties of a newpH-responsive polymer containing amino acid residues. Polymer, 2002.43(15): p. 4285-4295 (reference), incorporated herein by reference inits entirety]. These bands were also observed for the monomerN,N-diallylaspartic acid hydrochloride (spectrum not shown). The resinCPZA also contains bands at 1304 cm⁻¹ and 1125 cm⁻¹ which have beenassigned in literature to asymmetric and symmetric bands of SO₂[SilviaMartinez-Tapia, H., et al., Synthesis and Structure ofNa₂[(HO₃PCH₂)3NH]1.5H₂O: The First Alkaline Triphosphonate. Journal ofSolid State Chemistry, 2000. 151(1): p. 122-129 (reference),incorporated herein by reference in its entirety]. In the unloadedcross-linked dianionic polyelectrolyte (CAPE), the C═O stretch shiftdramatically to 1578.7 cm⁻¹ and 1406.8 cm⁻¹ for asymmetric and symmetricvibrations respectively because it is now in COO⁻ form. After Pb²⁺adsorption an appreciable increase in the intensity and broadness of theCOO⁻ bands is noted [D. Kolodynska, Z.H.a.S.P.-P, FT-IR/PAS Studies ofCu(II)-EDTA Complexes Sorption on the Chelating Ion Exchangers. ActaPhysica Polonica A, 2009. 116(3): p. 340-343 (reference), incorporatedherein by reference in its entirety].

Example 6 Effect of pH on the Adsorption of Pb²⁺ by CAPE

The relationship between the initial pH of solution and the percentageremoval of Pb²⁺ is depicted in FIG. 2. In the pH range of 3-9, the Pb²⁺uptake was monitored by contacting the resin with 40 mgL⁻¹ lead (Pb²⁺)solution for 15 min at room temperature. The percentage Pb²⁺ removalinitially increased from 91.5% to 95.8% as the pH increased from 3 to 6.Further increase of pH to 9 saw a gradual decrease of Pb²⁺ uptake aspercentage Pb²⁺ dropped to 94.9%. It has been established that solutionpH plays a critical role in metal ion adsorption process due to itsinfluence on both the nature of the metal ions in solution and the stateof the functional groups on the surface of the adsorbents [Plazinski, W.and W. Rudzinski, Modeling the Effect of pH on Kinetics of Heavy MetalIon Biosorption. A Theoretical Approach Based on the Statistical RateTheory. Langinuir, 2008. 25(1): p. 298-304 (reference), incorporatedherein by reference in its entirety]. Studies have identified threeforms of lead species: Pb²⁺, Pb(OH)⁺ and Pb(OH)² in the pH range2.0-8.0. The distribution of these species as calculated by MINEQLsoftware shows that Pb²⁴ is the preponderant species at pH between 1 and6 and that its hydrolysis to Pb(OH)⁺ and Pb(OH)² starts as pH increaseswhile Pb(OH)² dominates at the pH higher than 6.0. [Dean, J. A., Lange'sHand Book of Chemistry. 1999; W. D. Schecher, D. C. M., MINEQL+: achemical equilibrium program for personal computers (Version 4.5).Environmental Research Software, Hallowell, Me., USA, 2001 (references),each incorporated herein by reference in their entirety”. Under low pH:(1) competition ensued between H+ and Pb²⁺ and (2) the functional groupson the surface of the resin are in protonated forms which do not favorcoordination with Pb²⁺ species as they are repelled by the electrostaticforce. Thus, the Pb²⁺ absorption capacity of the resin is decreased[Liu, W.-J., et al., Adsorption of lead (Pb) from aqueous solution withTypha angustifolia biomass modified by SOCl2 activated EDTA. ChemicalEngineering journal, 2011, 170(1): p. 21-28 (reference), incorporatedherein by reference in its entirety]. As the pH is increased, thiscompetition reduces and the functional groups on the resin are becomingless protonated, thereby making them more available for coordinationwith Pb²⁺. This accounts for increased percentage removal that peaked pH6.0. Beyond pH 6.0, there is deprotonation as basicity increases, andthe functional groups are in anionic forms that should encourage greatercoordination with Pb²⁺. However, this did not increase the percent Pb²⁺removal as lead are now being hydrolyzed into Pb(OH)⁺ and Pb(OH)²thereby reducing the amount of free Pb²⁺ available for complexation. Therest of the adsorption experiments were carried out at pH 6.0.

Example 7 Effect of Initial Concentration on the Adsorption of Pb²⁺ byCAPE

FIG. 3 depicts the effect of initial concentration of Pb²⁺ on thepercentage Pb²⁺ removal by cross-linked dianionic polyelectrolyte(CAPE). It can be seen that the percentage removal for the Pb²⁺ rapidlyincreases from 0 min to about 25 min contact time and thereafter slowlyuntil it reaches equilibrium. This is due to the fact that theadsorption kinetic depends on the surface area of the adsorbent which islargely uncovered at the start of the experiment (0 contact time).Therefore, the rate of adsorption at the early time increases rapidlyuntil it reaches a point where the remaining fewer adsorption sites arecompeted for by lead ions. Hence the rate of adsorption slows down untilit reaches equilibrium [Aroua, M. K., et al., Real-time determination ofkinetics of adsorption of lead(II) onto palm shell-based activatedcarbon using ion selective electrode. Bioresource Technology, 2008.99(13): p. 5786-5792 (reference), incorporated herein by reference inits entirety]. This is situation is observed for all the initialconcentration of lead ranging from 10-100 ppm. The effect of the amountof adsorbate was also investigated and it was observed that theadsorption capacity of CAPE increases as the concentration of Pb²⁺ israised from 10 up till 100 ppm, as seen in FIG. 4.

Example 8 Adsorption Kinetics

The dynamics of the interaction at the solid-solution interface duringthe adsorption of Pb²⁺ from aqueous solution can be described in termsof the following kinetic models:

Pseudo First-Order (Lagergren) Kinetics

Pseudo first-order relates the adsorption rate of solute to adsorptioncapacity of the adsorbent. The linear form of the equation is given byEquation (3) [S. Lagergren, K. S., Vetenskapsakad, Handl., 1898. 24: p.1-39 (reference), incorporated herein by reference in its entirety].

$\begin{matrix}{{\log\left( {q_{e} - q_{t}} \right)} = {{\log\mspace{14mu} q_{e}} - \left( \frac{k_{1}t}{2.303} \right)}} & (3)\end{matrix}$where q_(e) and q_(t) are the amounts of Pb²⁺ adsorbed in (mg g⁻¹) atequilibrium time and at any time, t, respectively and k₁ is the firstorder rate constant in (h⁻¹). A linear plot of log (qe−qt) versus tyields a straight line for the pseudo first-order kinetics from which k₁and q_(e,cal) are calculated. The combined Lagergren plots for thevarious initial concentrations of Pb²⁺ used for the kinetic studies whenadsorbent amount is kept at 1.5 gL⁻¹. The kinetic parameters extractedfrom these plots are displayed in Table 1. Judging from the fittings, itcan be seen that there is generally good linearity of Lagergren Pseudofirst-order plots as R² ranges from 0.95 to 0.99 for all the plots(Table 1). The rate constants k₁ are determined to vary between 2.7 to16.38 h⁻¹. There is a general downward trend of k₁ as initialconcentrations increase. However, considering the equilibrium adsorptioncapacities reveal a sharp disagreement between the experimentallyobserved q_(e,exp) and that derived from Largergren pseudo first-orderplots, q_(e,cal 1), as shown in Table 1. This is understood to mean thateven though, the model fits Pb²⁺ adsorption data quite fairly, it is notsuitable for estimation of q_(e,cal 1) as it is not a true first orderequation where the intercept of the plot of log (q_(e)−q_(t)) versus tshould be equal to log q_(e) as in Equation (3) [Ho, Y. S. and G, McKay,Comparative sorption kinetic studies of dye and aromatic compounds ontofly ash. Journal of Environmental Science and Health, Part A, 1999.34(5): p. 1179-1204 (reference), incorporated herein by reference in itsentirety].

TABLE 1 Adsorption kinetic parameters for Lagergren models. Pseudo 1storder Pseudo 2nd order Concentration q_(e,exp) k₁ q_(e,cal) q_(e,cal) k₂h^(a) (ppm) (mg g⁻¹) (h⁻¹) (mg g⁻¹) R² (mg g⁻¹) (g mg⁻¹ h⁻¹) (mg g⁻¹h⁻¹) R² 10 6.63 16.4 0.606 0.9854 6.70 55.7 2.50 × 10³ 1.000 20 13.310.1 0.908 0.9450 13.3 56.7 1.00 × 10⁴ 1.000 40 26.5 2.70 1.595 0.956226.3 8.10 5.60 × 10³ 0.9999 60 39.2 6.93 8.383 0.9541 39.4 3.23 5.01 ×10³ 1.000 100 59.4 4.97 3.964 0.9640 59.5 2.82 9.98 × 10³ 1.000^(a)Initial adsorption rate, h = k₂q_(e) ²Pseudo Second-Order Kinetics

The failure of Lagergren first-order kinetic model to correctly estimatethe equilibrium adsorption capacity q_(e) drives us to pseudosecond-order kinetic model for analysis of the dynamics. The linear formof the pseudo second order model can be written as:

$\begin{matrix}{\frac{t}{q_{t}} = {\frac{1}{k_{2}q_{e}^{2}} + \frac{t}{q_{e}}}} & (4)\end{matrix}$Here, k₂ (g mg⁻¹ h⁻¹) is pseudo second order rate constant, q_(e) andq_(t) are the adsorption capacities at equilibrium and at any time trespectively [Ho, Y. S. and G. McKay, Pseudo-second order model forsorption processes. Process Biochemistry, 1999. 34(5): p. 451-465(reference), incorporated herein by reference in its entirety]. A plotof t/q_(t) against t, gave linear relationship allowing for thecalculation of q_(e) and k₂ as displayed in table 1. The initialadsorption rate is also presented as h−k₂q_(e) ². As can be seen fromTable 1, pseudo second-order model gave an excellent fitting for theadsorption data with the square of regression coefficient of unity forall the experiments Interestingly, as can also be seen in Table 1, thecalculated equilibrium adsorption capacities, q_(e,cal 2), show verynice agreement with the experimentally observed, q_(e,exp) Minceva, M.,L, Markovska, and V. Meshko, Removal of Zn 2+, Cd 2+ and Pb²⁺ frombinary aqueous solution by natural zeolite and granulated activatedcarbon. 2007. Vol. 26. 2007 (reference), incorporated herein byreference in its entirety].Intra-Particle Diffusion

The Weber-Morris intraparticle diffusion was used as a model to evaluatethe diffusion contribution of Pb²⁺ adsorption within the resin. A plotof q_(t) against t^(1/2) as represented by Equation (5) should give astraight line if the mechanism of the adsorption is controlled by thediffusion of the adsorbate ions within the particle in the pore of theadsorbent [W. J. Weber. J. C. M., Eng. Div. Am. Soc. Civ. Eng., 1963.89: p. 31-60; Annadurai, G., R.-S. Juang, and D.-J. Lee, Use ofcellulose-based wastes for adsorption of dyes from aqueous solutions.Journal of Hazardous Materials, 2002, 92(3): p. 263-274 (references),each incorporated herein by reference in their entirety]. The diffusionprocess may take place in the external, macropore and micropore surfacesleading to multi-linear plots suggesting that other processes might playroles in the adsorption [Vergili, I., et al., Study of the Removal ofPb(II) Using a Weak Acidic Cation Resin: Kinetics, Thermodynamics,Equilibrium, and Breakthrough Curves. Industrial & Engineering ChemistryResearch, 2013. 52(26): p. 9227-9238; Wu, F. C., R. L. Tseng, and R. S.Juang, Adsorption of Dyes and Phenols from Water on the ActivatedCarbons Prepared from Corncob Wastes, Environmental Technology, 2001.22(2): p. 205-213 (references), each incorporated herein by reference inits entirety”.q _(t) =k _(id) t ^(1/2) +x _(i)  (5)where q_(t) is the adsorption capacity at any time t, k_(id) is theintraparticle diffusion rate constant (mg/g hr.) and x_(i) is a constantthat takes into account the boundary layer thickness. It can be seenfrom table 1 that the diffusion model exhibits three distinct parts forthe duration of the study at different temperatures. The effects oftemperature on the Weber-Morris parameters are displayed in fable 2. At298K, from the start of the process, the first and second straightportions of the curve with steep slopes reflect the easy diffusion ofPb²⁺ ion inside the macropores of the cross-linked dianionicpolyelectrolyte (CAPE) resin while the third part, indicates the slowdiffusion within the micropore. It is informative to note that the thirdpart shows a gradual approach to equilibrium which, is an indication ofa concentration dependent diffusion process [Rengaraj, S., et al.,Adsorption characteristics of Cu(II) onto ion exchange resins 252H and1500H: Kinetics, isotherms and error analysis. Journal of HazardousMaterials, 2007, 143(1-2): p. 469-477 (reference), incorporated hereinby reference in its entirety]. This result is in agreement withpseudo-second order kinetic model. Experiments conducted at 313 and 333Kshow similar behavior but with gradual decrease in k_(id) and theirregression coefficients, as shown in Table 2.

TABLE 2 Intraparticle diffusion parameters. Intraparticle diffusionmodel Temperature k_(id) x_(i) (K) (mg g⁻¹ h⁻¹) (mg g⁻¹) R² 298 2.1224.7 0.9812 313 0.998 25.15 0.9807 333 0.413 25.14 0.9804Adsorption Isotherm Models

The basic assumptions in Langmuir isotherm are that the adsorption is amonolayer type on a homogeneous surface where the adsorption at one siteis completely independent of the other [Langmuir, I., The Adsorption ofGases on Plane Surfaces of Glass, Mica and Platinum. Journal of theAmerican Chemical Society, 1918. 40(9): p. 1361-1403 (reference),incorporated herein by reference in its entirety] It can be expressed inlinear form as Equation (6) follows:

$\begin{matrix}{\frac{C_{e}}{q_{e}} = {\frac{C_{e}}{Q_{m}} + \frac{1}{Q_{m}b}}} & (6)\end{matrix}$where Q_(m) is the quantity of adsorbate required to form a singlemonolayer on a unit mass of the adsorbent (mg g⁻¹), Q_(e) is the amountadsorbed on a unit mass of adsorbent (mg g⁻¹) at equilibriumconcentration C_(e) (mgL⁻¹) and b is an equilibrium constant that takescare of the apparent energy of adsorption. A plot of C_(e)/q_(e) againstC_(e) yielded a straight line in agreement with Langmuir isotherm givingthe isotherm parameters as depicted in Table 3.

Comparisons of adsorption capacities from the Langmuir isotherm of thecurrent adsorbent for the removal of Pb²⁺ with those of other adsorbentsreported in the literature is displayed in Table 4. All the referenceslisted in Table 4 are incorporated by reference in their entireties.Based on the reported Pb²⁺ adsorption capacities, it: can be concludedthat the CAPE resin of the present disclosure compares favorably againstand is superior to these adsorbents.

Additional analysis was also made by using the dimensionless equilibriumparameter R_(L) which is a measure of the favorability of adsorption.R_(L) values are found to range from 0.09 to 0.90, which means that itis favorable adsorption process,

TABLE 3 Isotherm constants for adsorption of Pb²⁺ on CAPE. AdsorptionIsotherms Isotherm Parameters R² Langmuir Q_(m) 64.5 mg g⁻¹ 0.9943 b2.07 mg⁻¹ Freundlich k_(f) 34.2 0.9323 n 2.23 Temkin A 34.8 Lg⁻¹ 0.9948b 221 J mol⁻¹

TABLE 4 Comparison of the adsorption capacity of the resin and those ofvarious adsorbents in literature for Pb²⁺ as computed by the linearLangmuir equation. Adsorption Capacity Sorbent Materials (mg/g)Reference Commercial silica 3.9 Chiron, N., R. Guilet, and E, Deydier,Adsorption of Cu(II) and Ph(II) onto a grafted silica: isotherms andkinetic model., Water Research, 2003. 37(13): P. 3079-3086 Zeolites:Chabazite 6.0 Ouki, S.K. and M. Kavannagh, Performance of NaturalZeolites for the Treatment of Mixed Metal- Contaminated Effluents. WasteManagement & Research, 1997. 15(4): p. 383-394. Activated carbon 6.68Mishra, P.C. and R.K. Patel, Removal of lead and zinc ions from water bylow cost adsorbents. Journal of Hazardous Materials, 2009. 168(1): p.319-325. Porous lignin-based 27.1 Li, Z., Y. Ge, and L. Wan, Fabricationsphere of a green porous lignin-based sphere for the removal of leadions from aqueous media. Journal of Hazardous Materials, 2015. 285(0);p. 77-83 Diethylenetriamine- 31.4 Shen, W., et al., Adsorption of Cu(II)bacterial cellulose and Pb(II) onto diethylenetriamine- bacterialcellulose. Carbohydrate Polymers, 2009. 75(1): p. 110-114.Ethylenediamine 50.0 Musyoka, S.M., et al., Synthesis, modifiedcellulose Characterization, and Adsorption Kinetic Studies ofEthylenediamine Modified Cellulose for Removal of Cd and Pb. AnalyticalLetters, 2011. 44(11): p. 1925-1936, Porous lignin 62.6 Li, Z., Y. Kong,and Y. Ge, Synthesis xanthate resin of porous lignin xanthate resin forPb2+ removal from aqueous solution. Chemical Engineering Journal, 2015.270(0): p. 229-234. Cross-linked 64.5 The present disclosure dianionicpolyelectrolyte (CAPE)Freundlich Isotherm

The Freundlich isotherm describes multilayer adsorption taking place ona heterogeneous surface. As in many systems, the heat of adsorptiondecreases with increasing extent of adsorption but this model unifiesthe energy [Freundlic, H. M. F., Uber die adsorption in losungen.Zeitschrift fur Physikalische Chemie (Leipzig), 1906. 57A: p. 385-470(reference), incorporated herein by reference in its entirety]. Thelinear form is given as Equation (7) as follows:

$\begin{matrix}{{\log\mspace{14mu} q_{e}} = {{\log\mspace{14mu} k_{f}} + {\frac{1}{n}\log\mspace{14mu} C_{e}}}} & (7)\end{matrix}$where k_(f) is the Freundlich constant and n is the heterogeneity factorwhich is a measure of deviation from linearity. Table 3 shows a high nof about 2 which is far from unity and R² of 0.93.Temkin Isotherm

The Temkin isotherm takes into consideration the interaction betweenadsorbate and adsorbent with the consequence that the heat of adsorptionof all the molecules in the layer decreases linearly with furthercoverage [M. J. Tempkin, V. P. J. T., V. Pyzhev, Acta Physiochim URSS;1940; p. 217-222 (reference), incorporated herein by reference in itsentirety]. The linear form is given as Equation (8) as follows:

$\begin{matrix}{q_{e} = {{\frac{RT}{b}\ln\mspace{14mu} A} + {\frac{RT}{b}\ln\mspace{14mu} C_{e}}}} & (8)\end{matrix}$where T is the absolute temperature in Kelvin (K), R is the molar gasconstant. (8.3.14 J mol⁻¹ K⁻¹), A represent the equilibrium bindingconstant (L g⁻¹) corresponding to maximum binding energy and b (Jmol⁻¹)is related to the heat of adsorption. The intercept and slope from theplot of qe versus log Ce enabled determination of A and b as displayedin Table 3. Judging from the high value of the coefficient ofdetermination R²=0.9948, it can be concluded that Temkin isotherm modeldescribe this equilibrium very well.

Example 9 Adsorption Activation Energy

The activation energy of adsorption can be computed from the Arrheniusequation presented as follows:

$\begin{matrix}{{\ln\mspace{14mu} k_{2}} = {\frac{E_{a}}{2.303\mspace{14mu}{RT}} + {constant}}} & (9)\end{matrix}$where E_(a) (kJmol⁻¹) is the activation energy, k₂ (g mg⁻¹ h⁻¹) is thesecond order rate constant as shown in table 1 and R is the molar gasconstant (8.314 J mol⁻¹ K⁻¹) and T is the temperature of the solution inKelvin. Plotting In k₂ against 1/T gave a linear relation whose slope(Ea/R) allowed us to determine activation energy of adsorption Table 5.From Table 5, the activation energy is 39.29 kJmol⁻¹, which is low andtherefore, is an indication of the favorability of the adsorptionprocess.

TABLE 5 Thermodynamic and kinetic parameters for Pb²⁺ adsorption onCAPE. Temp ΔG ΔH ΔS E_(a) (K) (kJ mol⁻¹) (kJmol⁻¹) (Jmol⁻¹ K⁻¹)(kJmol⁻¹) 298 −11.1 313 −9.41 −43.9 −110 39.3 333 −7.20

Example 10 Thermodynamics of Adsorption

Studies on adsorption thermodynamics have employed the ratio ofq_(e)/C_(e) as the distribution constants K in the Vant-Hoff equation toderive the adsorption thermodynamic parameters [Sheha, R. R. and A. A.El-Zahhar, Synthesis of some ferromagnetic composite resins and theirmetal removal characteristics in aqueous solutions. Journal of HazardousMaterials, 2008. 150(3): p. 795-803 (reference), incorporated herein byreference in its entirety]. The equation is modified to give Equations(10) and (11) as follows:

$\begin{matrix}{{\ln\left( \frac{q_{e}}{C_{e}} \right)} = {{- \frac{\Delta\; H}{RT}} + \frac{\Delta\; S}{R}}} & (10)\end{matrix}$ΔG=ΔH+TΔS  (11)

where, all the letters and symbols have their usual meanings. Thethermodynamic parameters extracted from the plot are shown in Table 5.From Table 3, it is evident that as temperature climbs up from 298 to333 K, the free energy change ΔG becomes less negative meaning that asan exothermic process, it is not favored at higher temperatures. The ΔHof −43.87 kJmol⁻¹ also corroborates this observation. The negativechange in entropy Δ4 of −110.11 J mol⁻¹ K⁻¹ suggest a significant fallin randomness at solution-solid interface during adsorption.

Example 11 SEM and EDX Analyses of Unloaded CAPE and Pb²⁺-Loaded CAPE

A scanning electron microscope (SEM) and energy dispersive X-ray (EDX)analysis was performed for examining the surface morphology and thestructure of the polymer before and after the adsorption, with imagesand spectra shown in FIGS. 5A-5D. The EDX spectrum recorded forlead-loaded polymer adsorption (FIG. 5B) indicates the maincharacteristic peaks for Pb²⁺ at 2.34 and 10.55 keV in addition, to thepeaks of the other elements of the polymer structure FIG. 5A. Thisconfirms the binding of Pb²⁺ to the surface of the polymer. SEM imagesare depicted in the FIGS. 5A and 5C.

Example 12 Treatment of Real Wastewater Samples

Samples of industrial wastewater were used to study the effect of realwater matrix and to evaluate practical application of the polymer. Thesamples were spiked with 0.0 and 20 mg/L Pb²⁺, and then treated withpolymer under the optimum conditions. Table 6 presents the ICP resultsof the analysis of wastewater sample and wastewater sample after beingtreated with the prepared polymer. It also presents the analysis ofwastewater sample after being spiked with 20 ppm Pb²⁺ and then treatedwith the prepared polymer. The results the real wastewater matrix. Thisindicates the high efficiency and capability of polymer to be regardedas a potential adsorbent for high efficient and renewable adsorbent forPb²⁺ ions from aqueous solutions.

TABLE 6 Comparison of Pb(II) concentrations in wastewater sample beforeand after the treatment with the polymer resin Original Original samplesample spiked with 20000 μgL⁻¹ Pb²⁺and Metal (μgL⁻¹) then treated withthe CAPE adsorbent Pb 0.453 357.2 Co 0.362 0.213 Cu 857.14 418.20 As8.14 3.351 Mo 36.12 10.4 Cd 1.17 0.087 Hg 213.11 133.4 MDL: the methoddetection limit

Thus, the foregoing discussion discloses and describes merely exemplaryembodiments of the present invention. As will be understood by thoseskilled in the art, the present invention may be embodied in otherspecific forms without departing from the spirit or essentialcharacteristics thereof. Accordingly, the disclosure of the presentinvention is intended to be illustrative, but not limiting of the scopeof the invention, as well as other claims. The disclosure, including anyreadily discernible valiants of the teachings herein, defines, in part,the scope of the foregoing claim terminology such that no inventivesubject matter is dedicated to the public.

The invention claimed is:
 1. A method for removing one or more heavymetals ions from an aqueous solution, comprising contacting the aqueoussolution with a copolymer to adsorb the heavy metal ions on thecopolymer, wherein the copolymer comprises: an aspartic acid derivative;a quaternary ammonium salt; and sulfur dioxide; wherein the copolymerhas a structure with a repeating unit represented by Formula 1 orFormula 2:

wherein: R₁-R₅ are each independently a hydrogen, a halide, anoptionally substituted methyl group, or an optionally substituted ethylgroup; in Formula 1, each W⁻ is F⁻, Cl⁻, Br⁻, NO₃ ⁻ or other suitablemonoanion other than OH⁻; in Formula 2, each M⁺ is Li⁺, Na⁺, K⁺ or othersuitable monocation other than H⁺; and n=9-19.
 2. The method of claim 1,wherein in the copolymer a quaternary ammonium salt cross-links onepolymer chain of the copolymer to another polymer chain of thecopolymer.
 3. The method of claim 1, wherein the aspartic acidderivative comprises a quaternary nitrogen atom.
 4. The method,copolymer of claim 1, wherein R₁ is a hydrogen.
 5. The method of claim1, wherein the copolymer is in a basic and anionic form.
 6. The methodcopolymer of claim 1, wherein the copolymer has a repeating unitrepresented by Formula 1, wherein: R₁-R₅ are each a hydrogen; and W⁻ isCl⁻.
 7. The method of claim 1, wherein the copolymer has a repeatingunit represented by Formula 2, wherein: R₁-R₅ are each a hydrogen; andM⁺ is Na⁺.
 8. The method of claim 1, wherein the copolymer has arepeating unit having a Pb²⁺ adsorption capacity of 50-100 mg g⁻¹ basedon a total weight of the copolymer.
 9. The method of claim 1, whereinthe copolymer has a concentration of 0.02-5.0 g L⁻¹ in the aqueoussolution.
 10. The method of claim 1, wherein the contacting is carriedout at a temperature of 10-100° C.
 11. The method of claim 1, whereinthe contacting is carried out at an aqueous sample pH range of 3 to 9.